The victims were shot to death within the span of about three hours in Brazil's biggest metropolitan area, law enforcement officers stated on Friday.

Alexandre de Moraes, head of the Sao Paolo State Public Safety Department, said police were trying to determine if Thursday night's shootings in the suburbs of Osasco, Barueri and Itapevi, on the outskirts of Sao Paulo, were related.

At least 15 people were killed in the shootings and seven injured, he told reporters, adding that these numbers were still considered preliminary.

Local authorities said most of the attacks appeared to follow a pattern. Gunmen with face masks arrived by car, got out of the vehicle and quickly shot their victims.

The motive of this unusual wave of violence was still unclear, Moraes was quoted by the newspaper "Folha de Sao Paolo" as saying.

"We will investigate if the killings were committed by drug traffickers, by off-duty police officers or were acts of vengeance for the recent killings of two police officers," he told reporters.

das/sgb (AP, AFP, dpa)